1. Field of the Invention
The present invention relates to a knock detecting apparatus adapted for use with internal combustion engine ignition timing control systems, etc., which is designed to function so that knock is detected from the vibrations produced both inside and outside the engine cylinders due to the cylinder pressure and the ignition timing is controlled to obtain the desired knock intensity.
2. Description of the Prior Art
It is well known in the art that there is a close correlation between the ignition timing and the cylinder pressure. When a mixture is exploded in the cylinder, in the absence of knocking there is no superposition on the cylinder pressure of any higher harmonic component (usually a frequency component in the frequency band which is determined by the cylinder bore diameter and the velocity of sound in the combustion and which is produced by the intermittent and rapid combustion), whereas when knocking occurs such higher harmonic starts to superpose on the cylinder pressure at around the maximum value of the cylinder pressure and this results in the generation of vibrations or sound outside the cylinders. Examination of the pressure signal generated inside the cylinders and the generation of vibrations or sound outside the cylinders shows that the beginning of knock (trace knock) starts to occur at an engine crank angle at which the cylinder pressure attains the maximum value and that as the knock is increased (to light knock and heavy knock) the higher harmonic starts to superpose considerably earlier (or on the ignition side) than the maximum cylinder pressure crank angle. In this case, it is considered that the frequency of the higher harmonic caused by the knock is linearly determined by the cylinder bore diameter and the velocity of sound in the combustion as mentioned previously and it is also believed that this frequency is produced only in a specified frequency band (usually in the range of 7 to 10 kHz). As a result, knock detecting apparatus known in the art are based on the principle that the desired knock detection can be performed by simply detecting the vibration produced outside an engine or by noting only specified frequencies and the detection of knock is performed in this way so as to control the ignition timing. This type of known apparatus has been found disadvantageous in that the detection accuracy is subject to deterioration depending on the operating conditions of the engine and there are certain operating conditions under which the detection of very weak trace knock is difficult. More specifically, where the detection of knock is performed in a frequency band ranging from 7 to 10 kHz, when the speed increases, the vibration noise (e.g., vibration noise due to the valve seating action) produced in the engine body increases with the resulting deterioration of the S/N ratio. Particularly, under high speed and load operation of the engine the effect of such vibration noise on the knock detection is so large that the detection of trace knock is almost impossible. If one dares to control the ignition timing despite the deteriorated S/N ratio, this will tend to cause an excessively heavy knock and sometimes damages to the engine body including melting of the spark plugs will be caused. In order that such damages may be prevented, generally it is unavoidable to stop controlling the ignition timing under the high speed and load operation. On the other hand, in the case of known apparatus designed to simply detect vibrations without noting any specified frequency band, the effect of such vibration noise is so large that the detection of trace knock is practically impossible and smooth control of the ignition timing cannot be expected. Investigations of knock detecting methods intended to overcome the deficiencies of the prior art methods showed that the occurrence of knock would not be limited to any single frequency band but takes place in a plurality of frequency bands and that the recognition of such plurality of frequency bands would make it possible to accurately detect the occurrence of knock even under high engine speed operation without any danger of the engine vibration noise being superposed on the cylinder pressure. FIG. 1 of the accompanying drawings shows the results obtained by mounting a piezoelectric element type vibration detector on the engine block and analyzing the vibration outputs indicative of the presence and absence of knock. In the Figure, designated by A and B are the vibration outputs (back noise) produced at the engine speeds of 1500 and 3000 rpm, respectively, under high load operation (WOT) without any knock. Designated by C is the vibration output produced at the engine speed of 3000 rpm under WOT with knock. It will be seen from the results that the knock occurred in the frequency band of 5 to 10 kHz (hereinafter referred to as a low frequency band) as well as in the band of 11 to 13 kHz (hereinafter referred to as a high frequency band) and that under high engine speed operation the effect of vibration noise will be reduced and the S/N ratio will be improved in the frequency band of 11 to 13 kHz. It will also be seen that the sensitivity of the low frequency is improved under the low engine speed operation. The vibrations of the high frequency band have the most tendency to occur at around the peak cylinder pressure and the vibrations of the low frequency band have the most tendency to occur after the peak cylinder pressure. These results conform excellently with the cylinder pressure waveforms showing that the knocking frequency cannot be determined linearly as believed in the past and there are some special combustion regions. Further, while there are certainly cases where such high and low frequency bands cannot be absolutely determined depending on the types of engines due to the frequency bands being influenced by the shape and conditions of the combustion chamber, it is absolutely true that knock occurs in a plurality of frequency bands.